fix(context-pivot): reset pending state on session start

[outlier27-cell]

Problem

A context_pivot pending in an old Session could keep the extension's in-memory compacting state armed after Pi started a replacement Session. The new Session then received an incorrect "already in progress" rejection.

Closes #746

Value

Context pivots are now scoped to their originating Session, so a replacement Session can begin eligible compaction immediately.

Approach

• Invalidate pending pivot generations and clear the in-progress flag on session_start.
• Add a lifecycle regression covering a replacement Session before the old callback settles.

Validation

• node --test --experimental-strip-types tests/extensions/context-pivot/index.test.ts
• bun run check
• Exact-head GitHub CI is tracked for the final cross-platform result.

Impact

No threshold, summary, or compaction semantics change within one Session. Only stale prior-Session state is discarded.

[outlier27-cell]

@tt-a1i 请审核此 PR。最新提交的 GitHub CI 已全部通过；会话切换待处理状态的回归已验证，现仅等待维护者审核。